1. Field of the Invention
The invention relates to umbrella frame and more particularly to a telescopic shank of a collapsible umbrella, the shank being made of fibre-reinforced plastic (FRP) having advantages including light weight.
2. Description of Related Art
Umbrellas are personal articles. An umbrella can be collapsible or non-folding. An umbrella comprises a shank, a rib assembly, and a canopy secured onto the rib assembly. A collapsible umbrella has a folding rib assembly and a telescopic shank. A non-folding umbrella has a shank of fixed length. Shank of an umbrella can be made of metal or fibre-reinforced plastic (FRP). Shank of a collapsible umbrella comprises an outer tube, an inner tube, and a moving mechanism mounted in the inner tube. Typically, the inner tubes of small umbrellas are made of metal. Large umbrellas (e.g., golf umbrellas) have shanks of larger diameter, and a large rib and stretcher assembly. Thus, they are heavy. It is labor consuming for a user carrying a large umbrella for long distance walk. Therefore, both the inner tube and the outer tube of shank of a large umbrella (e.g., gold umbrella) are made of FRP.
U.S. Pat. No. 6,866,053 entitled “Beach Umbrella Having Telescopic Shank” discloses a telescopic shank made of FRP wherein both the inner tube and the outer tube have eccentric section so that they can be fastened together. However, its manufacturing is difficult because eccentric tubes cannot be easily molded due to different wall thicknesses. Further, fibers are not uniform in length. Furthermore, polymers of FRP are not uniformly mixed in the manufacturing process. Thus, precision of both the inner tube and the outer tube is poor with looser tolerances. As a result, both the inner tube and the outer tube tend to malfunction after a time of use.
Further, metal shank can be machined but FRP shank cannot be machined. Thus, positioning of the inner tube and the outer tube and positioning of the inner tube and the moving mechanism are impossible for FRP shank. Thus, no disclosure of telescopic FRP shank of an umbrella is available. To the contrary, improvements with respect to weigh reduction of a large umbrella (e.g., golf umbrella) are made continuously. As a result, length reduction of an umbrella shank made of FRP is not possible as the present inventor is aware.
For reducing weight of umbrella shank, aluminum alloy or other light metal materials can be used as raw material for manufacturing umbrellas. However, its manufacturing cost is several times of that of FRP frame. That is, its market survivability is low due to competitiveness. Thus, how to devise a light umbrella having a telescopic FRP shank is always desirable among manufacturers in the art. The invention discussed below aims at providing a collapsible umbrella having a telescopic shank made of FRP with all drawbacks associated with conventional large umbrellas being eliminated.